Double vanity plumbing options

Hello everyone, I'm sorry for posting another double vanity question, but this forum truly is the best place to go for all things plumbing related and as I am not a plumber this was the best place to go. I have been able to find answers to all of my questions so farâ€¦which leads me to this oneâ€¦which happens to be my first post. I think I have it figured out but I would like to make sure I am thinking about it correctly. I live in Georgia and our plumbing code our county uses is the International Plumbing Code. I am doing a guest bathroom remodel on a house built in 1970 which originally had one vanity initially installed in the guest bathroom, but it was also plumbed with the sink from the master (on the left side, other side of the wall). I want to convert this right side single sink set up to a dual vanity set up. The total number of fixtures on this 3 in vent line are at most three (there is on downstairs sink that may be vented) and once the new sink added would be four.

The way I see it there are three options,

Option 1: just hook up the drain from the new sink to the existing drain line. (I think this is incorrect b/c of siphoning issues with the other sink, but since this would be the easiest option I wanted to see if I was missing something.)

Option 2: Hook up the drain from the new sink to the existing drain line and then run a 1 Â½ vent line straight up through the roof.

Option 3: Hook up the drain from the new sink to the existing drain line and then connect into the stack (see picture).

I think option 2 and/or 3 would meet code but Iâ€™m not sure, also any other ideas/options would be very helpful. Thanks again

None of your options meet code, and ALL could cause siphoning of one sink or the other. #2 and #3 are exactly the same and could siphon the left hand sink because of water flowing past its connection without a vent. To do it correctly, you would have to insert a new tee lower than the existing one, then either pipe both sinks to it, using a back to back fixture fitting, or the new sink into it using the piping design from either 2 or 3 with a tee in the vertical riser.

Thanks for the advice so far, I really appreciate it. I've redone some of the drawings so please check them out. The first two correspond to the two option of what I think HJ was describing above. The third option was to raise both sink drains and tie back into existing drain. The fourth option ties into existing drain but inserts a vent in between the two sinks. Do these options meet IP code? Also am considering redoing the entire cast iron stack upward w/ pvc since it would be relative easy to do. Final question, when attaching pvc to existing cast iron on a vertical run such as this are the shielded fernco's (the one's completely encased metal) the best way to attach the pvc to cast iron? If so where would be the optimal place to cut and attach.

Placement and design is a dance...some of it depends on the height and configuration of the vanity (drawers, shelves, etc.) AND the depth of the sink. Then, you have to work around existing plumbing. Since you have to put the trap under the sink, and the trap arm needs to go into the drain without going up, there will be some height that becomes too high. Some people are using kitchen cabinet height vanities (nominally 36") verses the old standard of 30" or so - that can make a big difference on where your trap arm can go.

I think I will go with the third option (raising the drain height). We are going from our old vanity height of 30" to our new vanity height of 35" so that gives me at least 5" + to work with so I'm thinking that is the way to go. If I do go that option and tie a pvc T into the existing cast iron what would be the best way to do that? I'm sure I will have to use a fernco couple (likely the shielded one). Would it be better to cut at the hub or above the hub of the cast iron? Thanks again.

They make donuts to transition to pvc that go into the hub. Not all CI hub sizes are anything but standard, so they make lots of sizes of donuts. Get the wrong one, and it's a pain to insert (hard to compress the stiff rubber), or it's loose and leaks. If you have enough room, and the pipe is in good condtion, (cutting CI can shatter pieces in bad condition) then a no-hub (shielded) fitting is the proper way to do it.

Thanks for the response. THe CI seems to be in pretty good shape. So I'm thinking cutting it would be ok without it disintigrating. If I go that route, what would be the best way to secure the CI pipe, riser clamps? One final thought, instead of doing the option we discussed, what about running a 1.5 in vent line between new sink and original sink up to the attic and placing a studor vent on it? What would be the pros and cons of that. It would be a simplier install. Thanks again for your help it is great to bounce different ideas off of experienced pro's.

There are special clamps designed to anchor CI. They need to be attached to solid framing. The connection from CI to pvc (or abs) is with a no-hub connector (reinforced banded rubber sleeve). Inspectors will usually not approve the use of an AAV when a viable atmospheric vent is available - IOW, I doubt they'd approve its use in this application. Plus, the extreme heat in a place like Georgia in the attic is likely to destroy the air seal, and then you'd be leaking sewer gasses into the attic which isn't good, either.

Adding a vent in between the two, depending on how it was done, could still leave the original one improperly vented. The 'break' or vent has to be between the trap arm and where it attaches to the drain - you can't run waste past the trap arm without it being vented at that point.

#1 and #2 are correct, but the horizontal drain line will be 2" pipe, (there are NO 1 1/2" double fixture fittings). #3 would raise the drains so high you would not likely be able to connect to the sinks properly and the line would have to be changed to 2" anyway because of the fixture fitting. #4 is just a variation of your original "improper" drains, and as Abraham Lincoln once said, "You can call a dog's tail a leg, but he will still only have four legs". Doing #3 will raise the drain openings at least 6 or 7 inches, (you would have to measure the "center to center" dimension when the double fitting is placed on a "long sweep" elbow. Your "vent tee" would have to be inserted at least 42" above the floor so that eliminates any discussion about where to make the cut, relative to the hub.

Forgive me if this sound like a stupid question, but what if in my last option, I were to move my vent to the right side/or equal with the new sink drain (it was my initial second option), would that not be circuit venting and would that not be acceptable?

You cannot run waste past a trap arm without it being vented at that point, or if you try, you'll siphon it dry. So, look at how the waste is flowing, where you're connecting things to it, and see if the vent is in the proper position.

None of your options meet code, and ALL could cause siphoning of one sink or the other. #2 and #3 are exactly the same and could siphon the left hand sink because of water flowing past its connection without a vent. To do it correctly, you would have to insert a new tee lower than the existing one, then either pipe both sinks to it, using a back to back fixture fitting, or the new sink into it using the piping design from either 2 or 3 with a tee in the vertical riser.

Click to expand...

Hi,
I realize that this thread is old but I had a followup question that I think I know the answer to, but thought I'd ask anyway. Using the OP's post #1 images as a reference, Could Option #3 be modified to include a vertical vent off the original sink (left black dot) to the (horizontal red) newly added vent line (obviously, my left sink is not directly on a stud)? Given the relative simplicity of this option and the fact that it did not come up in the thread, I am assuming that it is not valid, but I wanted to be sure as it would be easier to implement for me.

Here is a modified version of the OP's photo. (green line is the subject of the question):

It would still have two sinks on a 1 1/2" drain, AND it would still raise the opening to the point that the sink would have to be raised higher than he would want it. Decades ago, like 60 years, there WAS a fitting that would have done that, but it was still for 2" pipe, not 1 1/2".

It would still have two sinks on a 1 1/2" drain, AND it would still raise the opening to the point that the sink would have to be raised higher than he would want it. Decades ago, like 60 years, there WAS a fitting that would have done that, but it was still for 2" pipe, not 1 1/2".